Liquid metal thermal storage system and method

ABSTRACT

Embodiments of the invention relate to systems and methods for storing thermal energy from working fluid heated by a high-temperature heat source and retrieving the thermal energy. The heated working fluid is in thermal communication with heat exchanger elements that can efficiently store thermal energy by, for example, phase change in one or more metal alloys.

PRIORITY CLAIM

This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/664,643 filed Jun. 26, 2012 entitled “LIQUID METAL THERMAL STORAGE SYSTEM AND METHOD” which application is incorporated herein in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate to systems and methods for storing thermal energy.

BACKGROUND OF THE INVENTION

Market dynamics and government regulations are leading to the development and deployment of next-generation energy technologies that are more cost-competitive, efficient, and sustainable than their predecessors. In particular, thermal transport and conversion play an important role in more than 90% of energy technologies, and there is opportunity to improve power generation, delivery, and consumption by developing thermal technologies.

For example, concentrated solar power (CSP), a thermal-electrical conversion technology, is promising for power plants because of its potential to meet cost, efficiency, and sustainability targets. CSP uses solar collectors (e.g., heliostats, parabolic troughs, linear Fresnel reflectors, etc.) to concentrate sunlight on solar receivers. The solar receivers contain working fluids (e.g., steam, oil, liquid salt, liquid alkali metal, gas, etc.) to collect and transfer heat to operate thermodynamic cycle engines (e.g., Rankine, Brayton, Stirling, etc.). The engines provide mechanical power to generators for emissions-free, utility-scale electricity generation.

There are four primary arrangements of solar collectors, solar receivers, and thermodynamic cycle engines in CSP power plants: Fresnel, parabolic trough, parabolic dish, and power tower. Fresnel, parabolic trough, and parabolic dish arrangements are generally considered low-temperature CSP as their operating temperatures are restricted to approximately 450° C. for Fresnel and parabolic trough, and approximately 750° C. for parabolic dish. In contrast, power tower arrangements enable higher operating temperatures because their solar collectors and receivers are more scalable. Power tower arrangements use dual-axis solar collectors that track the sun and reflect sunlight onto one or more central solar receivers atop towers. The working fluid from the solar receivers flows through piping to operate thermodynamic cycle engines that are typically located at ground level.

Thermal energy storage (TES) is beneficial to CSP and other next-generation thermal technologies. Utilities generally require that CSP and other power plants provide dispatchable power on the order of 75% of the year. The sun is only above the horizon for 50% of the year on average, so CSP needs TES to meet the utilities' requirement by producing and storing excess thermal energy when the sun is shining, and converting it into electricity when the sun is below the horizon or obscured (e.g., by clouds). Some sources indicate TES can increase CSP capital utilization from approximately 30% to 60%.

TES can be accomplished by storing energy as sensible heat or latent heat (or some combination thereof). There are two primary implementations of sensible heat storage in CSP. In the first implementation, the sensible heat of the working fluid is used to collect, transfer, and store heat. The working fluid can be stored in separate hot and cold tanks, or by using a thermocline configuration, in which a single tank has a colder, denser bottom layer and a hotter, less dense upper layer. In the second implementation, the sensible heat of the working fluid is transferred to a separate and distinct TES system that stores energy as sensible heat. Oil and liquid salts are common in sensible heat TES systems employed in CSP power plants.

Latent heat storage is distinct from sensible heat storage because it does not result in temperature change, and is experienced through phase change such as solidification/melting and condensation/vaporization. There are two fundamental advantages of latent heat storage relative to sensible heat storage, particularly for CSP. The first advantage is that latent heat storage is an isothermal process. Isothermal energy storage results in a working fluid at near constant temperature with which to operate thermodynamic cycle engines at optimum conditions. The second advantage is that latent heat stores significantly more energy than sensible heat per unit of storage medium. The amount of energy stored as specific heat is determined by the product of the specific heat and the temperature change. For example, the specific heat of water is 1 cal/g-° C. so one gram of water releases 1 calorie of heat if the temperature is lowered by 1° C. By comparison, the latent heat of fusion (i.e., solidification) of water is approximately 80 cal/g. Therefore, sensible heat storage at quasi-isothermal conditions (i.e., 1° C. temperature change) near 0° C. requires eighty-fold more water than latent heat storage.

The current state-of-the-art TES systems in CSP power plants based on a power tower arrangement use sensible heat storage in liquid salts, which poses several challenges. First, sensible heat capacities of liquid salts are relatively low compared to latent heats of metals and metal alloys. For example, the specific heat capacity of sodium and potassium nitrate mixtures, known as solar salts, is approximately 0.35 cal/g-° C.; in contrast, the latent heat of fusion of silicon is approximately 430 cal/g. Therefore over one thousand-fold more liquid salt is required to store the same amount of energy at quasi-isothermal conditions (i.e., 1° C. temperature change) as silicon. Second, liquid salts have relatively poor heat transfer properties compared to metals and metal alloys: the thermal conductivity of solar salts is on the order of 1 W/m-° C. versus 237 W/m-° C. for aluminum. In addition, solidification of solar salts around heat exchangers inhibits natural convection, which further deteriorates heat transfer.

The operating temperatures of TES systems (and downstream thermodynamic cycle engines) are also important to CSP and other next-generation thermal technologies because they directly affect conversion efficiency. Carnot's theorem states that the maximum efficiency of a heat engine is determined by the temperature difference of the hot and cold reservoirs between which it operates. Operating TES systems at the highest temperature possible results in maximum efficiency because ambient air is typically the cold reservoir and its temperature is not easily controlled. The TES systems based on liquid salts or oils described above are impractical at high temperatures. Energy storage in working fluid (e.g., steam, supercritical CO₂, air, etc.) at high temperatures is difficult because of very high pressures, and oils and solar salts break down at approximately 400° C. and 570° C., respectively, although some fluoride salts are stable between approximately 350° C. and 850° C. New high-temperature TES systems are beneficial to utilizing thermodynamic cycle engines with operating temperatures in excess of 575° C. (e.g., supercritical H₂O, CO₂ Rankine, Brayton, etc.) for CSP and other next-generation thermal technologies.

INCORPORATION BY REFERENCE

The following references are incorporated herein by reference in their entireties:

-   U.S. Provisional Application No. 61/276,269, filed Sep. 10, 2009, by     Arlon J. Hunt, entitled “Liquid Metal Thermal Storage System”; -   U.S. patent application Ser. No. 12/878,896, filed Sep. 9, 2010, by     Arlon J. Hunt, entitled “Liquid Metal Thermal Storage System”; -   U.S. Pat. No. 4,512,388, issued Apr. 23, 1985, by Terry D. Claar et     al., entitled “High-Temperature Direct-Contact Thermal Energy     Storage Using Phase-Change Media”; -   Simensen “Comments on the Solubility of Carbon in Molten Aluminum”     Metallurgical Transactions A Vol. 20A January 1989, p. 191; -   Winter, Sizmann, @ Vant-Hull, Solar Power Plants, Chapter 6,     Springer, Verlag 1991; -   Guthy and Makhlouf “The aluminum-silicon eutectic reaction:     mechanisms and crystallography” Journal of Light Metals Vol. 1, No.     4, November 2001, pp. 199-218. -   Department of Energy Advanced Research Projects Agency-ENERGY (DoE     ARPA-E)—HEATS Funding Opportunity Announcement (issued Apr. 20,     2011, retrieved Jul. 2, 2011) -   Metallurgical Transactions A Volume 15A, March 1984-467 -   Metals Handbook, 8th ed. (American Society for Metals, Metals Park,     Ohio, 1973), Vol. 8, p. 263 -   Energy Conyers. Manag., 47 (2006), 2211

SUMMARY OF THE INVENTION

Embodiments of the system and method of the invention include a system and a method for storing and retrieving thermal energy from a working fluid heated by a high-temperature heat source. The system comprises an insulated channel containing heat exchanger elements, wherein the high-temperature working fluid is passed through the channel containing the heat exchanger elements that are in thermal communication with one or more metal alloys that melt at specific temperatures between approximately 577° C. and 1414° C. to store thermal energy; and wherein a working fluid to be heated is passed through said channel where the thermal energy stored is given up by the metal alloy(s).

Other embodiments include a system for storing and retrieving thermal energy from a working fluid heated by a high-temperature heat source comprising: first heat exchanger elements containing one or more metal alloys that melt at specific temperatures between approximately 577° C. and 1414° C. to store thermal energy; a second insulated channel that is adapted to accept a working fluid that is heated by the high-temperature heat source, with said second channel in thermal communication with the first heat exchanger elements; and a third insulated channel that is adapted to accept a fluid to be heated by the first heat exchanger elements, with said third channel in thermal communication with the first heat exchanger elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, forthcoming, and other aspects will be readily appreciated by the skilled artisan from the following descriptions of the preferred embodiments of the invention when read in conjunction with the accompanying drawings.

FIG. 1 a is a schematic side view illustration of the parallel tube array for the pressurized TES embodiment of the invention.

FIG. 1 b is a schematic top view illustration of the parallel tube array for the pressurized TES embodiment of the invention.

FIG. 1 c is a schematic cross-section view illustration of the parallel tube array for the pressurized TES embodiment of the invention.

FIG. 2 a is a schematic side view illustration of the perpendicular tube array for the pressurized TES embodiment of the invention.

FIG. 2 b is a schematic top view illustration of the perpendicular tube array for the pressurized TES embodiment of the invention.

FIG. 2 c is a schematic cross-section view illustration of the perpendicular tube array for the pressurized TES embodiment of the invention.

FIG. 3 a is a schematic side view illustration of the pebble bed array for the pressurized TES embodiment of the invention.

FIG. 3 b is a schematic top view illustration of the pebble bed array for the pressurized TES embodiment of the invention.

FIG. 3 c is a schematic cross-section view illustration of the pebble vessel bed array for the pressurized TES embodiment of the invention.

FIG. 4 a is a schematic perspective view illustration of sample axial fins on vessels in the form of tubes of an embodiment of the invention.

FIG. 4 b is an end view illustration of sample axial fins on vessels in the form of tubes of an embodiment of the invention.

FIG. 4 c is a schematic perspective view illustration of sample radial fins on vessels in the form of tubes of an embodiment of the invention.

FIG. 5 a is a schematic side view illustration of a planar configuration for the unpressurized TES embodiment of the invention.

FIG. 5 b is a schematic cut-away perspective view illustration of a concentric configuration for the unpressurized TES embodiment of the invention with high-temperature working fluid inside the TES annulus.

FIG. 5 c is a schematic cut-away perspective view illustration of a concentric configuration for the unpressurized TES embodiment of the invention with high-temperature working fluid outside the TES annulus.

FIG. 5 d is a schematic side view illustration of sealed hollow tube containing a relatively small amount of working fluid in the unpressurized TES embodiment of the invention.

FIG. 5 e is a schematic side view illustration of a planar configuration for the electrical unpressurized TES embodiment of the invention.

FIG. 5 f is a schematic cut-away perspective view illustration of a concentric configuration for the electrical unpressurized TES embodiment of the invention with high-temperature working fluid inside the TES annulus.

FIG. 5 g is a schematic cut-away perspective view illustration of a concentric configuration for the electrical unpressurized TES embodiment of the invention with high-temperature working fluid outside the TES annulus.

FIG. 6 a is a schematic side view illustration of a ceramic or superalloy vessel in an embodiment of the invention.

FIG. 6 b is a schematic side view illustration of a clad graphite or vitreous carbon vessel in an embodiment of the invention.

FIG. 7 a is a schematic perspective view illustration of a vessel of an embodiment of the invention during charging; the vessel is in the form of a tube, oriented parallel to flow, and divided into compartments of cascading Al/Si metal alloy composition.

FIG. 7 b is a schematic perspective view illustration of a vessel of an embodiment of the invention during charging; the vessel is in the form of a tube, oriented parallel to flow, and divided into compartments of cascading Al/Si metal alloy composition.

FIG. 7 c is similar to FIG. 5 a with the vessel provided with compartments.

FIG. 8 a shows a schematic illustration of a CSP power plant based on a power tower arrangement.

FIG. 8 b shows the components in a CSP power plant based on a power tower arrangement incorporating an embodiment of the invention.

FIG. 8 c shows the flow of working fluid during TES charging for an embodiment of the invention in a CSP power plant based on a power tower arrangement.

FIG. 8 d shows the flow of working fluid during TES discharging for an embodiment of the invention in a CSP power plant based on a power tower arrangement using an upcomer.

FIG. 8 e shows the flow of working fluid during TES discharging for an embodiment of the invention in a CSP power plant based on a power tower arrangement using a bypass.

FIG. 9 a shows a schematic side view illustration of an embodiment of the invention supported by a horizontal section in a CSP power plant based on a power tower arrangement.

FIG. 9 b shows a schematic cross-section view illustration of an embodiment of the invention supported by struts in a CSP power plant based on a power tower arrangement.

FIG. 10 a shows a schematic illustration of the pressurized TES embodiment of the invention in a wind or hydro power plant.

FIG. 10 b shows a schematic illustration of the electrical pressurized TES embodiment of the invention in a wind or hydro power plant.

FIG. 11 is an equilibrium phase diagram for pure aluminum-silicon metal alloys.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is illustrated, by way of example and not by way of limitation, in the figures of the accompanying drawings in which like references indicate similar elements. References to embodiments in this disclosure are not necessarily to the same embodiment, and such references mean at least one. While specific embodiments are discussed, it is understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without departing from the scope and spirit of the invention.

In the following description, numerous specific details will be set forth to provide a thorough description of the invention. However, it will be apparent to those skilled in the art that the invention and embodiments thereof may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the invention.

Embodiments of the invention relate to the use of phase change in metal alloys to store (and release) energy as latent heat. Certain metal alloys store very large amounts of energy as latent heat at temperatures suitable for operating high-temperature thermodynamic cycle engines for CSP and other next-generation thermal technologies.

In a first embodiment of the invention, at least one metal alloy is contained in at least one vessel located within an insulated channel through which working fluid is circulated. This embodiment is referred to as the pressurized TES embodiment. The pressurized TES embodiment is charged by flowing high-temperature working fluidthrough the insulated channel to heat and induce phase change (e.g., melting) in the metal alloy(s) contained in the vessel(s). The high-temperature working fluid is heated by solar receivers or other heat sources such as electrical resistance heating wires that can be powered by, for example, wind or hydro power. The pressurized TES embodiment is discharged by flowing working fluid to be heated through the same insulated channel to cool and induce phase change (e.g., solidification) in the metal alloy(s) contained in the vessel(s).

In a second embodiment of the invention, at least one metal alloy is contained in at least one vessel located outside at least one insulated channel through which working fluid is circulated. Heat transfer rods provide thermal communication between the vessel(s) and the insulated channel(s). This embodiment is referred to as the unpressurized TES embodiment. The unpressurized TES embodiment is charged by transferring heat via heat transfer rods from high-temperature working fluid flowing through an insulated channel to heat and induce phase change (e.g., melting) in the metal alloy(s) contained in the vessel(s). The high-temperature working fluid is heated by solar receivers or other heat sources such as electrical resistance heating wires that can be powered by, for example, wind or hydro power. The unpressurized TES embodiment is discharged by transferring heat via the same or different heat transfer rods to working fluid to be heated flowing through the same or a different insulated channel to cool and induce phase change (e.g., solidification) in the metal alloy(s) contained in the vessel(s).

In a third embodiment of the invention, at least one metal alloy is contained in at least one vessel located within an insulated channel through which working fluid is circulated. Electrical resistance heating wires wrapped around the vessel(s) supply energy to the metal alloy(s) through resistive heating. This embodiment is referred to as the electrical pressurized TES embodiment. The electrical pressurized TES embodiment is charged by flowing electrical current through the electrical resistance heating wires to heat and induce phase change (e.g., melting) in the metal alloy(s) contained in the vessel(s). The electrical pressurized TES embodiment is discharged by flowing working fluid to be heated through the insulated channel to cool and induce phase change (e.g., solidification) in the metal alloy(s) contained in the vessel(s).

In a fourth embodiment of the invention, at least one metal alloy is contained in at least one vessel located outside an insulated channel through which working fluid is circulated. Electrical resistance heating wires wrapped around the vessel(s) supply energy to the metal alloy(s) through resistive heating, and heat transfer rods provide thermal communication between the vessel(s) and the insulated channel. This embodiment is referred to as the electrical unpressurized TES embodiment. The electrical unpressurized TES embodiment is charged by flowing electrical current through the electrical resistance heating wires to heat and induce phase change (e.g., melting) in the metal alloy(s) contained in the vessel(s). The electrical unpressurized TES embodiment is discharged by transferring heat via the heat transfer rods to working fluid to be heated flowing through the insulated channel to cool and induce phase change (e.g., solidification) in the metal alloy(s) contained in the vessel(s).

In any of the embodiments, there is a wide choice of available metal alloys. Metal alloys are formed from a combination of two or more elements with phase change temperatures determined by the fraction of each element present. The phase change temperatures are chosen to correspond to the desired operating temperatures of CSP and other next-generation thermal technologies.

Metal alloys composed of aluminum, silicon, and optional trace elements, referred to as Al/Si metal alloys, can be chosen in the embodiments because of their desirable thermodynamic and heat transfer characteristics relative to other thermal storage media such as liquid salts or oils. The phase change temperatures of Al/Si metal alloys free of trace elements range from approximately 577° C. to 1414° C. depending on the relative composition of aluminum and silicon. Therefore Al/Si metal alloys enable the delivery of working fluid to thermodynamic cycle engines across a broad temperature range, in contrast to the limited temperature range enabled by liquid salts or oils.

Another advantage of Al/Si metal alloys relative to liquid salts or oils are their high latent heats. While the latent heat of fusion of aluminum is relatively high compared to other pure metals, the latent heat of fusion of silicon is among the highest known of any material. As described above, the latent heat of fusion of silicon is approximately 430 cal/g; in contrast, the specific heat capacity and latent heat of fusion of solar salts (i.e., liquid salts) are approximately 0.35 cal/g-° C. and 100 cal/g, respectively.

Al/Si metal alloys also have very good heat transfer properties that enable faster charging/discharging than liquid salts or oils. The thermal conductivity of aluminum just below its melting temperature of 660° C. is approximately 237 W/m-° C., and the thermal conductivity of pure silicon is 149 W/m-° C. at 27° C. These values are considerably higher those of liquid salts or oils; for example, the thermal conductivity of solar salts is on the order of 1 W/m-° C.

The materials of the vessels containing the metal alloy(s) are considerations in the embodiments. The vessel materials can be ceramics such as alumina, magnesia, or zirconia, superalloys (i.e., metal alloys with strong performance at high temperatures) such as Inconel and Waspalloy, or clad graphite or vitreous carbon. Graphite and vitreous carbon must be clad in ceramics or superalloys if the working fluid is air or another oxidizing gas (e.g., carbon dioxide) because carbon allotropes oxidize at high temperatures. Cladding can be accomplished by chemically bonding ceramics or superalloys to graphite or vitreous carbon, or slip fitting ceramics or superalloys around graphite or vitreous carbon. To be specific, slip fitting refers to creating ceramic or superalloy vessels that are geometrically similar to, but slightly larger than, the graphite or vitreous carbon vessels. The graphite or vitreous carbon vessels are then placed inside the ceramic or superalloy vessels, which are sealed closed (e.g., through welding) so no oxidizing gases can penetrate. The graphite or vitreous carbon inner vessels can be in physical contact with the ceramic or superalloy vessels, or separated by small spacers of ceramics, superalloys, graphite, or vitreous carbon that help accommodate thermal expansion and mitigate any potentially adverse interactions between the inner and outer vessel materials. The void space in the sealed ceramic or superalloys vessels (i.e., the space not filled by the metal alloy(s), graphite or vitreous carbon, or optional spacers) can be filled with helium, argon, or another non-reactive gas to promote heat transfer and help balance the pressure differential inside and outside the sealed ceramic or superalloy containers.

The heat transfer rods of the unpressurized TES embodiment can be made from solid ceramics, superalloys, or clad graphite or vitreous carbon. They can also be sealed hollow tubes made from ceramics, superalloys, or clad graphite or vitreous carbon that contain a relatively small amount of working fluid. The working fluid in the sealed hollow tubes should change phase from liquid to vapor at a temperature above the phase change temperature of the metal alloy(s) with which they are in thermal communication for tubes communicating with the high-temperature working fluid. The working fluid is vaporized in the lower ends of the tubes by the high-temperature working fluid, and condenses in the upper ends of the tubes that are embedded in the metal alloy(s) in the insulted vessel(s). For communicating with the working fluid to be heated, the working fluid in the sealed closed hollow tubes should change phase from liquid to vapor at a temperature below the phase change temperature of the metal alloy(s) with which they are in thermal communication. The working fluid is vaporized in the lower ends of the tubes that are embedded in the metal alloy(s) in the insulted vessel(s), and condenses in the upper ends of the tubes by the working fluid to be heated. Considerations for the heat transfer rods of the electric unpressurized TES embodiment are the same as those of the unpressurized TES embodiment that communicate with the working fluid to be heated.

The electrical resistance heating wires in the electrical pressurized and unpressurized TES embodiments are also considerations. The heating wire materials should have high electrical resistivity, high melting temperatures, high corrosion (i.e., oxidation) resistance, and other desirable properties. By way of example, Nichrome or Constantan are candidate heating wire materials.

The preferred embodiments of the invention are illustrated in the context of a Brayton thermodynamic cycle engine operating in a CSP power plant based on a power tower arrangement. The skilled artisan will readily appreciate, however, that the systems and methods disclosed herein apply in a number of other high-temperature TES contexts. For example, the embodiments can be incorporated into wind or hydro power plants to convert electricity into heat, and store the heat for future thermal-electrical conversion in a thermodynamic cycle engine. The pressurized and unpressurized TES embodiments can be charged by heating working fluid with electrical resistance heating wires, and the electrical pressurized and unpressurized TES embodiments can also be charged by heating the vessel(s) and metal alloy(s) directly with electrical resistance heating wires.

The arrays of one or more heat exchange elements in the preferred pressurized TES embodiment and electrical pressurized TES embodiment are discussed first. It is to be understood that the structures, such as the vessels, the tubes and the spheres, may, from time to time, be referred to as heat exchanger elements generally. Three examples of arrays of heat exchanger elements are: (1) an array of one or more vessels in the form of tubes parallel to the working fluid flow, referred to as the parallel tube array; (2) an array of one or more vessels in the form of tubes perpendicular to the working fluid flow, referred to as the perpendicular tube array; and, (3) an array of one or more vessels in the form of spheres in a pebble bed, referred to as the pebble bed array. FIG. 1 a to FIG. 1 c are side, top, and cross-section views of a 3-tube parallel tube array; FIG. 2 a to FIG. 2 c are side, top, and cross-section views of an 8-tube perpendicular tube array; and FIG. 3 a to FIG. 3 c are side, top, and cross-section views of a 7-sphere pebble bed array. FIG. 1 a to FIG. 1 c show the channel 100 through which working fluid flows, the vessels (heat exchanger elements) 102 containing the metal alloy(s), the insulation 104 surrounding the channel, and electrical resistance heating wires 105 (for the electrical pressurized TES embodiment); FIG. 2 a to FIG. 2 c show the channel 200 through which working fluid flows, the vessels (heat exchanger elements) 202 containing the metal alloy(s), the insulation 204 surrounding the channel, and electrical resistance heating wires 205 (for the electrical pressurized TES embodiment); FIG. 3 a to FIG. 3 c show the channel 300 through which working fluid flows, the vessels (heat exchanger elements) 302 containing the metal alloy(s), the insulation 304 surrounding the channel, and electrical resistance heating wires 305 (for the electrical pressurized TES embodiment). The vessels can have fins or other appendages or structures that maximize heat transfer, particularly in the parallel and perpendicular tube arrays; the appendages or structures on the vessels 402 could have axial (FIG. 4 a and FIG. 4 b) or radial orientations (FIG. 4 c). The vessels in FIG. 4 a, FIG. 4 b, and FIG. 4 c are shown without electrical resistance heating wires, but vessels with electrical resistance heating wires can also have fins or other appendages or structures.

Vessels in the preferred unpressurized TES embodiment are illustrated next in FIG. 5 a, FIG. 5 b, and FIG. 5 c. FIG. 5 a is a side view of one vessel 502 containing the metal alloy(s) 506, the high-temperature insulated channel 500 h, the low-temperature insulated channel 500 l, and the insulation 504 surrounding the channels in a planar configuration. FIG. 5 b is a side view of one vessel 502 containing metal alloy(s) 506, the high-temperature insulated channel 500 h, the low-temperature insulated channel 500 l, and the insulation 504 surrounding the channels in a concentric configuration; FIG. 5 c is identical to FIG. 5 b except the high-temperature insulated channel 500 h and low-temperature insulated channel 500 l are switched. In FIG. 5 a, FIG. 5 b, and FIG. 5 c, the metal alloy(s) 506 thermally communicate with the insulated channels 500 h and 500 l through heat transfer rods 508 and 510, respectively. The heat transfer elements are made from solid ceramics, superalloys, or clad graphite or vitreous carbon, or from sealed hollow tubes made from ceramics, superalloys, or clad graphite or vitreous carbon that contain a relatively small amount of working fluid. If the heat transfer rods 508 are sealed hollow tubes, then the working fluid is selected to change phase from liquid to vapor at a temperature above the phase change temperature(s) of the metal alloy(s) 506. By way of example, the working fluids within the heat transfer rods 508 could be magnesium (vaporization temperature of 1090° C.) or lithium (vaporization temperature of 1342° C.). The working fluid is vaporized in the lower ends of the heat transfer rods 508 by absorbing heat from the high-temperature working fluid flowing through the insulated channel 500 h; the working fluid then releases heat by condensing in the upper ends of the heat transfer rods 508 that are embedded in the metal alloy(s) 506. If the heat transfer rods 510 are sealed hollow tubes, then the working fluid is selected to change phase from liquid to vapor at a temperature below the phase change temperature(s) of the metal alloy(s) 506. The working fluid is vaporized in the lower ends of the heat transfer elements 510 by absorbing heat from the metal alloy(s) 506; the working fluid then releases heat by condensing in the upper ends of the heat transfer rods 510 in the insulated channel 500 l containing working fluid to be heated. By way of example, the working fluids within the heat transfer elements 510 could be potassium (vaporization temperature of 760° C.) or sodium (vaporization temperature of 883° C.). FIG. 5 d shows a sample sealed hollow tube heat transfer rod 508 (or 510) containing a relatively small amount of working fluid 512.

Vessels in the preferred electrical unpressurized TES embodiment are illustrated in FIG. 5 e, FIG. 5 f, and FIG. 5 g. The vessels in the preferred electrical unpressurized TES embodiment are similar to those in the preferred unpressurized TES embodiment, but include the electrical resistance heating wires 505 and lack the high-temperature insulated channel 500 h and associated heat transfer elements 508 of FIG. 5 a, FIG. 5 b, and FIG. 5 c.

The preferred embodiments can be oriented vertically, horizontally, or slantingly. A vertical orientation of the preferred embodiments is especially attractive for two reasons. First, it enables the preferred embodiments to be incorporated into vertical piping from solar receivers to avoid additional cost associated with piping, insulation, etc. Second, it enables natural convection within the metal alloy(s) to promote beneficial mixing during phase change. For the preferred unpressurized TES embodiment and the electrical preferred unpressurized TES embodiment, the heat transfer elements always have a vertical component to promote natural convection.

The vessels in the preferred embodiments are numbered, sized, spaced, and oriented to simultaneously maximize metal alloy volume and heat transfer, and minimize pressure drop in the working fluid. This is accomplished by considering working fluid flow and temperature, which can be characterized by one or many dimensionless numbers such as the Reynolds number, Nusselt number, and Grashof number. The Reynolds, Nusselt, and Grashof numbers are determined by the properties and temperature of the working fluid and metal alloy(s), and the characteristic dimensions of the vessels. The Reynolds number is the ratio of inertial forces to viscos forces; lower Reynolds numbers indicate laminar flow characterized by smooth, constant fluid motion, while higher Reynolds numbers indicate turbulent flow characterized by chaotic eddies, vortices, and other flow instabilities. Pressure drop in the working fluid does not monotonically increase or decrease as a function of the Reynolds number, and therefore the number, size, spacing, and orientation of the preferred vessels are selected to target Reynolds numbers in the laminar flow regime and the turbulent flow regime. The Nusselt number is the ratio of convective heat transfer to conductive heat transfer across a boundary (e.g., the boundary between the working fluid and the vessels); lower Nusselt numbers indicate convection is limiting heat transfer, while higher Nusselt numbers indicate conduction is limiting heat transfer. The number, size, spacing, and orientation of the preferred vessels are selected to target higher Nusselt numbers so convective heat transfer into and out of the working fluid, and into and out of the metal alloy(s), is not limiting TES charging and discharging. The Grashof number is the ratio of buoyancy force to viscous force, and is primarily used in understanding the degree of natural convection in a fluid; lower Grashof numbers indicate relatively less natural convection, while higher Grashof numbers indicate relatively more natural convection. The number, size, spacing, and orientation of the preferred vessels are selected to target higher Grashof numbers so natural convection in the metal alloy(s) supports heat transfer and TES charging and discharging.

The choice of metal alloy(s) in the preferred embodiments is influenced by many factors including, by way of example: phase change temperatures and kinetics, latent heats, thermal conductivity, expansion/contraction during phase change and associated natural convection, stability during cycling, chemical reactivity with vessel materials and heat transfer elements, effects of contaminants, and current and future prices of the metal alloy(s).

In the preferred embodiments, Al/Si metal alloys are chosen because of their desirable thermodynamic and physical characteristics. Some metal alloys with suitable melting temperatures for TES, such as aluminum (melting temperature of 660° C. and latent heat of fusion of 95 cal/g) and magnesium (melting temperature of 650° C. and latent heat of fusion of 88 cal/g), do not have sufficiently high latent heats for commercialization. Other pure metals with high latent heats are rare, expensive, radioactive, toxic, or have impractically high or low melting temperatures. However, Al/Si metal alloys have tunable phase change temperatures and latent heats of fusion that are well-suited for high-temperature TES used in conjunction with a Brayton thermodynamic cycle engine operating in a CSP power plant based on a power tower arrangement or other next-generation thermal technologies.

Al/Si metal alloys form a eutectic system with two distinct phase changes from solid to liquid. The first phase change from solid to mushy (i.e., partially solid, partially liquid) occurs at the solidus temperature, which depends on the type and quantity of trace elements present. For example, the solidus temperature is approximately 577° C. for pure Al/Si metal alloys free of trace elements, but approximately 572° C. for Al/Si metal alloys with trace amounts of alkali metals. The second phase change from mushy to liquid occurs at the liquidus temperature, which depends on the relative composition of aluminum and silicon as well as the type and quantity of trace elements present. For pure Al/Si metal alloys free of trace elements, the liquidus temperature is equal to the solidus temperature (i.e., phase change is from solid to liquid) for the composition of approximately 87.4 wt. % aluminum and 12.6 wt. % silicon. This composition is termed the eutectic composition, and abbreviated as AlSi12. Compositions with relatively less silicon content than the eutectic composition are termed hypoeutectic compositions or hypoeutectics, and those with relatively more silicon content than the eutectic composition are termed hypereutectic compositions or hypereutectics. Liquidus temperatures for hypoeutectics monotonically decrease from 660° C., the melting temperature of pure aluminum, to 577° C. for the eutectic composition as silicon content increases. Liquidus temperatures for hypereutectics monotonically increase from 577° C. for the eutectic composition to 1414° C., the melting temperature of pure silicon, as silicon content increases. The equilibrium phase diagram for pure Al/Si metal alloys free of trace elements is shown in FIG. 11 with the solid, mushy, and liquid phases, and the solidus and liquidus temperatures indicated. The presence of trace elements tends to decrease the solidus and liquidus temperatures, and increase the silicon content of the eutectic composition.

The latent heat storage of Al/Si metal alloys is dependent on phase change, and therefore it is beneficial to understand the solid, mushy, and liquid phases as functions of composition. The solid phase is actually a solid solution: an aluminum and silicon mixture at the eutectic composition (i.e., AlSi12) serves as the solvent, and excess aluminum for hypoeutectics or silicon for hypereutectics serves as the solute. The ratio of AlSi12 to excess aluminum or silicon is dependent on composition. For example, the composition of 50 wt. % aluminum and 50 wt. % silicon, abbreviated as AlSi50, is approximately 57.2 wt. % AlSi12 and 42.8 wt. % excess silicon because the 50 wt. % aluminum requires 7.2 wt. % silicon to form AlSi12.

When transitioning from solid to mushy at the solidus temperature, AlSi12 melts, but excess aluminum or silicon does not change phase. Therefore the mushy phase is solid aluminum in molten AlSi12 for hypoeutectics, and solid silicon in molten AlSi12 for hypereutectics. When transitioning from mushy to liquid at the liquidus temperature, the solid aluminum or silicon melts to form a truly liquid phase. In the reverse transitions, excess liquid aluminum or silicon solidifies at the liquidus temperature (i.e., liquid to mushy), and AlSi12 solidifies at the solidus temperature (i.e., mushy to solid).

The solid, mushy, and liquid phases as functions of composition directly affect the latent heat storage of Al/Si metal alloys in the embodiments. The energy stored as latent heat during phase change from solid to mushy at the solidus temperature is attributable to melting AlSi12, which has a latent heat of fusion of approximately 134 cal/g. Noncoincidentally, linear interpolation between the latent heats of fusion of aluminum at 95 cal/g and silicon at 430 cal/g suggests a latent heat of fusion of approximately 137 cal/g for AlSi12. The energy stored as latent heat during phase change from mushy to liquid at the liquidus temperature is attributable to melting the excess aluminum fraction for hypoeutectics, and melting the excess silicon fraction for hypereutectics. For example, the latent heat of fusion for AlSi50 at the liquidus temperature is approximately 184 cal/g based on 42.8 wt. % excess silicon with the latent heat of fusion of silicon at 430 cal/g.

Therefore the amount of energy stored as latent heat and the temperature at which the energy is stored in Al/Si metal alloys can be tuned by selecting a specific composition of aluminum and silicon in the embodiments. The liquidus temperature can be tuned from 577° C. for the eutectic composition to 1414° C., the melting temperature of pure silicon. Likewise, the latent heat of fusion at the liquidus temperature can be tuned from approximately 95 cal/g to 134 cal/g for hypoeutectics (i.e., from the latent heat of fusion of pure aluminum to that of the eutectic composition), and from 134 cal/g to 430 cal/g for hypereutectics (i.e., from the latent heat of fusion of the eutectic composition to that of pure silicon). This enables the delivery of working fluid at near constant temperature from 577° C. to 1414° C. to thermodynamic cycle engines for operation at optimum conditions. The embodiments can also store latent heat at several temperatures by varying the composition of aluminum and silicon spatially. For example, each of the embodiments can have one or more vessels, and each vessel can have one or more compartments containing different compositions of aluminum and silicon. This ‘cascading’ strategy allows the embodiments to maximize the temperature difference and heat transfer between the Al/Si metal alloys and working fluids.

Al/Si metal alloys also have very good heat transfer properties, are containable, and have low price points. The thermal conductivities of pure aluminum and pure silicon at 25° C. are approximately 250 W/m-° C. and 149 W/m-° C., respectively. Al/Si metal alloys also transfer heat through natural convection because of expansion/contraction during phase change. Additionally, the expansion/contraction that promotes natural convection in Al/Si metal alloys is not expected to rupture the vessels in the preferred embodiments. Pure aluminum expands during phase change from solid at 2.7 g/cm³ to liquid at 2.4 g/cm³. In contrast, pure silicon contracts during phase change from solid at 2.4 g/cm³ to liquid at 2.6 g/cm³. Therefore mushy is the least dense phase for hypereutectics because it is solid silicon in molten AlSi12. Depending on composition, mushy is between 0% and 10% less dense than the solid or liquid phases. The mushy and liquid phases change shape to accommodate any vessel and mushy contracts upon solidification to prevent rupturing. From a practical perspective, Al/Si metal alloys are inexpensive, readily available, and stable because no known degradation paths exist.

Specifically, hypereutectic Al/Si metal alloys are chosen in the preferred embodiments. Hypereutectics have two distinct advantages over hypoeutectics. First, they have a much broader liquidus temperature range: their liquidus temperature range includes that of hypoeutectics from 575° C. to 660° C., and also liquidus temperatures from 660° C. to 1414° C. Second, they have much higher latent heats of fusion at liquidus temperatures owing to their excess silicon content. Furthermore, the preferred embodiments are designed to store energy at the liquidus temperatures of hypereutectic Al/Si metal alloys. The energy stored as latent heat at the liquidus temperature is greater than at the solidus temperature for compositions above approximately 34 wt. % silicon (abbreviated as AlSi34) because of silicon's extremely high latent heat of fusion. Additionally, storing energy at the liquidus temperatures of hypereutectics avoids potential complications associated with solidification of the Al/Si metal alloys.

In the preferred embodiments, the materials of the vessels are simultaneously compatible with high-temperature hypereutectic Al/Si metal alloys and oxidizing working fluid (e.g., air, carbon dioxide, etc). Therefore the vessel materials of the preferred embodiments are ceramics, superalloys, or clad graphite or vitreous carbon. The preferred unpressurized TES embodiment and electrical unpressurized TES embodiment also include heat transfer rods, which can be made from solid ceramics, superalloys, or clad graphite or vitreous carbon. They can also be sealed hollow tubes made from ceramics, superalloys, or clad graphite or vitreous carbon that contain a relatively small amount of working fluid; the selection of working fluid in the sealed closed hollow tubes is discussed above. In FIG. 6 a, the vessels 602 is composed of ceramics or superalloys contain metal alloy(s) 606. In FIG. 6 b, the graphite or vitreous carbon inner vessel 602 a containing metal alloy(s) 606 is protected from oxidizing working fluid by the ceramic or superalloy outer vessel 602 b and fit with optional spacers 612. In the preferred electrical pressurized and unpressurized TES embodiments, the electrical resistance heating wires are made from heating wire materials that have high electrical resistivity, high melting temperatures, high corrosion (i.e., oxidation) resistance, and other desirable properties.

A representative preferred pressurized TES embodiment is a vertically-oriented parallel tube array of vessels containing hypereutectic Al/Si metal alloys; the vessels can have fins or other appendages or structures that maximize heat transfer such as those shown in FIG. 4 a and FIG. 4 b, or FIG. 4 c. The vessels are numbered, sized, spaced, and finned to target Reynolds numbers in the laminar and turbulent flow regimes, higher Nusselt numbers, and higher Grashof numbers to maximize Al/Si metal alloy volume and heat transfer, and minimize pressure drop in the working fluid. Each vessel is comprised of 2 or more compartments, with adiabatic spacers between compartments to prevent thermal spillover. FIG. 7 shows the channel 700 through which working fluid flows, a representative vessel divided into 3 compartments 702 a, 702 b, and 702 c containing the Al/Si metal alloy(s), the Al/Si metal alloys themselves 706 a, 706 b, and 706 c, the adiabatic spacers 714, and the insulation 704 surrounding the channel. Each compartment contains a different composition of hypereutectic Al/Si metal alloy selected such that the liquidus temperature of a given compartment is higher than the compartment below it, which results in hypereutectic compositions that cascade from more excess silicon to less excess silicon. Additionally, the compositions are selected such that the liquidus temperatures of all compartments are lower than the temperature of the high-temperature working fluid as it enters the channel, but higher than the temperature of the working fluid to be heated as it enters the channel. FIG. 7 a and FIG. 7 b illustrate such a cascading strategy: the liquidus temperature of the top compartment 702 a is higher than the middle compartment 702 b, which is in turn higher than the bottom compartment 702 c. The liquidus temperatures of compartments 702 a, 702 b, and 702 c are lower than the temperature of the high-temperature working fluid as it enters the channel 700 from the top during charging, but higher than the temperature of the working fluid to be heated as it enters the insulated channel 700 from the bottom during discharging (i.e., T_(working,700,charging)>T_(liquidus,702a)>T_(liquidus,702b)>T_(liquidus,702c)>T_(working,700,discharging)). For example, the aluminum-silicon composition of metal alloy 706 a could be AlSi50 with a liquidus temperature of approximately 1030° C., the composition of metal alloy 706 b could be 55 wt. % aluminum and 45 wt. % silicon (i.e., AlSi45) with a liquidus temperature of approximately 980° C., and the composition of metal alloy 706 c could be 60 wt. % aluminum and 40 wt. % silicon (i.e., AlSi40) with a liquidus temperature of approximately 930° C. if the high-temperature working fluid enters at 1050° C. and the working fluid to be heated enters at 400° C.

This cascading strategy parallels a counter-current heat exchanger to maximize heat transfer. During charging as shown in FIG. 7 a, high-temperature working fluid enters through the top and thermally communicates first with the compartment containing the hypereutectic with the highest liquidus temperature, and then thermally communicates with one or more compartments containing hypereutectics with increasingly lower liquidus temperatures. During discharging as shown in FIG. 7 b, working fluid to be heated enters through the bottom and thermally communicates first with the compartment containing the hypereutectic with the lowest liquidus temperature, and then thermally communicates with one or more compartments containing hypereutectics with increasingly higher liquidus temperatures.

FIG. 7 c is similar to FIG. 5 a with similar numbering with the chamber 502 divided into compartments 512 a, 512 b, and 513 c and with each compartment including different Al/Si alloys 516 a, 516 b, and 516 c. Adiabatic spacers 514 can divide the compartments as in the manner shown in FIGS. 7 a and 7 b. A cascading strategy and/or counter-current flow strategy similar to that demonstrated for the embodiments of FIGS. 7 a and 7 b can be employed in the embodiment of FIG. 7 c.

During charging, high-temperature working fluid flows from top to bottom through the channel because it originates in solar receivers above the TES system and is destined for a gas turbine below. During discharging, flow through the channel is reversed and working fluid to be heated enters through the bottom and exits through the top to maintain the counter-current parallel. The flow directions also result in natural convection during phase change from mushy to liquid. Phase change from mushy to liquid occurs more quickly at the top of each vessel than at the bottom during charging, and phase change from liquid to mushy occurs more quickly at the bottom of each vessel than at the top during discharging. Mushy is less dense than liquid so it will rise, creating a natural convection loop that promotes beneficial mixing within each vessel.

The TES system of the representative preferred pressurized TES embodiment is placed in the downcomer connecting a solar receiver to a turbine in a Brayton thermodynamic cycle engine operating in a CSP power plant based on a power tower arrangement. In the power tower arrangement shown in FIG. 8 a, the solar collectors 804 c reflect sunlight onto a central solar receiver 804 located atop a tower 804 t. Working fluid flows between the central solar receiver 804, TES system 806, and Brayton thermodynamic cycle engine 804 b (including the compressor 800, recuperator 802, and the turbine 808) through piping 804 p. FIG. 8 b provides additional detail on the solar receiver, TES system, Brayton thermodynamic cycle engine, and piping: the compressor 800, recuperator 802, solar receiver 804, TES system 806, and turbine 808 are shown, as well as the flow valve_(a) 810 a, flow valve_(b) 810 b, flow valve_(s) 810 c, compressor-recuperator piping 801, recuperator-valve_(a) piping 803 a, valve_(a)-receiver piping 803 b known as the upcomer, receiver-valve_(b) piping 805 a, valve_(b)-valve_(c) piping 807 b known as the bypass, valve_(b)-TES piping 805 c and TES-valve_(c) piping 807 a collectively known as the downcomer, valve_(s) turbine piping 807 b, valve_(s) valve_(a) piping 807 c, valve_(a)-turbine piping 807 d, tubine-recuperator piping 809 a, and recuperator exhaust piping 809 b.

FIG. 8 c shows the CSP power plant during charging. Compressed working fluid flows from the compressor 800 to the recuperator 802 through the compressor-recuperator piping 801, is preheated in the recuperator 802, and flows through the recuperator-valve_(a) piping 803 a, valve_(a) 810 a, and the upcomer (i.e., valve_(a)-receiver piping 803 b) to the solar receiver 804. High-temperature working fluid from the solar receiver 804 then flows through receiver-valve_(b) piping 805 a to valve_(b) 810 b, which directs some or all of the high-temperature working fluid through the TES system 806 via the downcomer (i.e., valve_(b)-TES piping 805 c and TES-valve, piping 807 a) and the balance of the high-temperature working fluid through the bypass (i.e., valve_(b)-valve, piping 807 b). At valve, 810 c, there are three options for the working fluids flowing through the downcomer 805 c/807 a and bypass 807 b to the turbine 808 in the preferred embodiments: if flow is present in both the downcomer 805 c/807 a and the bypass 807 b, (1) the working fluids can be mixed before entering the valve_(s) turbine piping 807 d; or (2) the working fluid exiting the TES system 806 can be vented before reaching valve, 810 c; or (3) if flow is only present in the downcomer 805 c/807 a, the working fluid from the TES system 806 enters the valve_(s) turbine piping 807 d. The working fluid from the valve_(s) turbine piping 807 d is then expanded through the turbine 808 to produce electricity. The bypass 807 b enables the CSP operator to separate charging the TES system from flowing working fluid through the turbine for electricity production.

FIG. 8 d and FIG. 8 e show two options of the CSP power plant during discharging. In FIG. 8 d, compressed working fluid flows from the compressor 800 to the recuperator 802 through the compressor-recuperator piping 801, is preheated in the recuperator 802, and flows through the recuperator-valve_(a) piping 803 a, valve_(a) 810 a, valve_(c)-valve_(a) piping 807 c, valve, 810 c, and TES-valve, piping 807 a to the TES system 806. Working fluid to be heated enters the TES system 806, is heated, and then flows through the valve_(b)-TES piping 805 c, valve_(b) 810 b, receiver-valve_(b) piping 805 a, solar receiver 804, valve_(a)-receiver piping 803 b, valve_(a) 810 a, and valve_(a)-turbine piping 807 d to the turbine 808 to produce electricity. This option, in which working fluid to be heated flows through the downcomer and upcomer, minimizes additional cost associated with piping, insulation, etc. by utilizing the upcomer.

In FIG. 8 e, compressed working fluid flows from the compressor 800 to the recuperator 802 through the compressor-recuperator piping 801, is preheated in the recuperator 802, and flows through the recuperator-valve_(a) piping 803 a, valve_(a) 810 a, valve_(s)-valve_(a) piping 807 c, valve, 810 c, and TES-valve, piping 807 a to the TES system 806. Working fluid to be heated enters the TES system 806, is heated, and then flows through the valve_(b)-TES piping 805 c, valve_(b) 810 b, bypass (i.e., valve_(b)-valve, piping 807 b), valve, 810 c, and valve_(s)-turbine piping 807 b to the turbine 808 to produce electricity. This option, in which working fluid to be heated flows through the downcomer and bypass, reduces the temperature requirements of the upcomer.

The TES system can be positioned at any point in the downcomer. If positioned at the base of the downcomer, it can be supported by resting on the horizontal section of the downcomer; if suspended above the base of the downcomer, it can be supported by struts or other structures made of ceramic or superalloy. FIG. 9 a shows the TES system 906 resting on the horizontal section of the channel 900 (i.e., downcomer) with surrounding insulation 904, and FIG. 9 b shows the TES system 906 supported by struts 916 composed of superalloy in the channel 900 with surrounding insulation 904. In the representative preferred pressurized TES embodiment, the TES system is positioned at the base of the downcomer to minimize the need for struts.

The preferred embodiments of the invention are also illustrated in the context of a wind or hydro power plant. In the embodiments, energy is converted from electricity to heat through resistive heating, and stored as heat in the metal alloy(s) for future thermal-electrical conversion in a thermodynamic cycle engine. FIGS. 10 a and 10 b show the channel 1000 through which working fluid flows, the vessels 1002 containing the metal alloy(s), the insulation 1004 surrounding the channel, the electrical resistance heating wires 1005, the wind or hydro power block 1001, and the thermodynamic cycle engine (for example, a Rankine or Brayton thermodynamic cycle engine) 1003. FIG. 10 a is consistent with the pressurized and unpressurized TES embodiments, with the electrical resistance heating wires 1005 located in the channel 1000 upstream of the vessels 1002. The working fluid in the channel 1000 is heated with the electrical resistance heating wires 1005 from energy produced by the wind or hydro power block 1001, and the heat stored in the vessels 1002 is subsequently used to operate the thermodynamic cycle engine 1003. FIG. 10 b is consistent with the electrical pressurized and unpressurized TES embodiments, with the electrical resistance heating wires 1005 wrapped around the vessels 1002. The vessels 1002 containing the metal alloy(s) are heated with the electrical resistance heating wires 1005 directly from energy produced by the wind or hydro power block 1001, and the heat stored in the vessels 1002 is subsequently used to operate the thermodynamic cycle engine 1003.

The foregoing description of the preferred embodiments of the present invention has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations can be apparent to the practitioner skilled in the art. Embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the relevant art to understand the invention. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

1. A system for storing and retrieving thermal energy from a fluid heated by a high temperature source comprising: a chamber containing heat exchanger elements, wherein the heated fluid is passed through the chamber containing the heat exchanger elements, which heat exchanger elements are in thermal communication with one or more metal alloys that melt at a specific temperature between about 577° C. and 1414° C. to store thermal energy; and wherein a fluid to be heated is passed through said chamber where the one or more metal alloys give up the thermal energy stored.
 2. A system for storing and retrieving thermal energy from a fluid heated by a high temperature source comprising: a first chamber containing one or more metal alloys that melt at a specific temperature between about 577° C. and 1414° C. to store thermal energy; a second chamber that is adapted to accept a fluid that is heated by the high temperature source, said second chamber in thermal communication with the first chamber; and a third chamber this is adapted to accept a fluid that is to be heated by the first chamber, said third chamber in thermal communication with the first chamber.
 3. A method using the system of claim
 1. 4. A power generation plant that uses the system of claim 1 as a source of energy.
 5. A power generation plant that uses the system of claim 1 as an alternative source of energy when a primary source of energy is unavailable.
 6. The system of claim 1 wherein said heat exchanger elements have a plurality of compartments with each compartment having one or more metal alloys that melt at a different specific temperature.
 7. The system of claim 2 wherein said first chamber has a plurality of compartments, with each compartment having one or more metal alloys that melt at a different specific temperature.
 8. The system of claim 1 wherein the heat exchanger elements have a plurality of compartments, with each successive compartment having a higher specific temperature at which the one or more metal alloys melt.
 9. The system of claim 2 wherein the first chamber has a plurality of compartments, with each successive compartment having a higher specific temperature at which the metal alloys melt.
 10. A method using the system of claim
 2. 11. A power generation plant that uses the system of claim 2 as a source of energy.
 12. A power generation plant that uses the system of claim 2 as an alternative source of energy when a primary source of energy is unavailable. 